The invention relates to machine vision systems, and particularly to an adaptive vision and guidance system for use in conjunction with robot systems.
Robots may be used in many applications to perform operations on a workpiece. For example a laser beam may be directed by a robot to perform various operations including welding, cutting, and milling. Many other operations such as conventional welding, water jet cutting, or bead blasting can also be performed by robots.
The guidance of a particular operation or function by a robot is made difficult by the fact that many robotic applications such as laser welding require very high accuracy, typically the order of plus or minus 5 to 7 mils (0.005 to 0.007 inch). Also, for many applications the path on which the operation is to be performed is not a straight line but may follow curves and complex patterns and abrupt changes of direction or width, in three dimensions. Errors are introduced not only by the robot itself but also by workpiece handling and positioning apparatus.
In many potential applications, therefore, the robotic systems are simply unable to track the desired path accurately enough over the entire path to perform the desired operation with an acceptable degree of quality.
To obtain an acceptable degree of quality, the position of the feature, orientation of the workpiece, and the path of the particular machine operation could be verified manually over the extent of the feature prior to the actual operation. However, manual verification eliminates many of the advantages provided by automation. Further, the error of robots over a large path is considerable, so that repeatability of the path of the end effector over the entire path, to the degree of accuracy normally required in welding operations, for example, is not possible. Accuracy, repeatability and predictability of robots can be very good over very short distances or times, e.g. the robot position one inch or one second from now relative to its present position. But this is not true over relatively great distances/times.
There have been many examples of pre-programmed automated welding without feedback, such as those used in automobile manufacturing. Such robotic applications are limited in achievable precision over long paths and require fairly precise locating. Also, features to be operated on can vary from one workpiece to the next, without ability of the robotic system to adapt. Poor quality products can result.
The ability of a robotic system to adapt to variations from one workpiece to the next is particularly important where the workpieces are used parts which require rework or repair. Where a robotic system is assembling or performing operations on new parts, the variations between the parts can be controlled or at least known to be within a certain range. On the other hand, where the robotic system must perform repair work, the required repair may change greatly from one workpiece to the next.
One solution to the problem of robot accuracy is to provide the robot with a guidance system which feeds position information back to the central robot control. This helps guide the robot in real time (or "pseudo real time"), with guidance based on inputs from sensors operating at normal robot process speeds. General Electric Company's MIG/TRAK system is an example of this type of robot guidance. The system employs feedback to a central robot to guide the robot along a seam. However, the speed at which a desired complex operation can be performed is severely limited due to the time required for the robot to respond to feedback information. The mass of the robot's components is one problem preventing acceptably fast acceleration and movement to respond fast enough for practical use in complex-movement operations. Also, such systems cannot relocate a seam or feature once it is lost. As a result, any time a feature is lost, operator intervention is required.
In many intricate welding operations such as those required in repair of aerospace components, prior to the present invention hand welding was the only practical procedure to achieve acceptable results. On original manufacture some automation has been possible with pre-programmed robotics, due to predictability of seam patterns, but systems for accomplishing welding of a complexly patterned crack or repair seam, for example in a jet engine casing, were not available. Some systems have employed vision systems for simple seam tracking, continuing until the seam or feature is lost. These systems had application in original production applications on simple parts, but have not been practically adaptable to more complex aerospace components.
Previous vision systems may be categorized as gray scale video or structured light systems. Gray scale systems obtain and process gray scale light/dark contrast information. Structured light systems use 3-D or topology information to detect a particular feature.
In structured light an external light source is used-typically a laser beam. The laser beam is impinged on the workpiece surface at one angle, and viewed with a camera from another angle. This gives a topology or resolution of the surface by the triangulation effect. With an appropriate detector, and with proper calibration, the surface topology can be determined electrically. In typical application of either type of system, a feature path is usually found by collecting data from the entire surface of the workpiece and then analyzing the data to detect the feature. Due to amount of data which must be processed with such a mode of operation, such prior vision systems have found limited practical use, at least in the application contexts of the present invention described below.